A half-bridge has two switching elements that are connected in series. The connection point of the switching elements constitutes the center point of the half-bridge. The center point of the half-bridge is switched alternately by the two switching elements to the positive pole and the negative pole of a direct-current power supply unit. A full bridge includes two half bridges, the center points of which are each switched at a desired frequency, that is to say, the basic frequency of the output signal of the full bridge, to the positive and negative pole of the direct-current power supply unit in a mutually opposite manner. An alternating-current load is arranged between those two center points.
The term “period” is used to refer to the period of time from connection of the positive pole of the direct-current power supply unit, disconnection of the positive pole, connection of the negative pole, disconnection of the negative pole, up to the time directly before the reconnection of the positive pole with respect to the alternating-current load by means of the switching elements. The order of the activation and deactivation of the switching elements within a period is referred to as the switching sequence.
In pulse-width control, the pulse width of the output signal is used to control the power of the output signal. The relationship of the period of time during which the alternating-current load is connected to the positive or negative pole relative to the total duration of a period is referred to as the pulse duty factor. A large pulse duty factor close to 100% accordingly results in waveforms wherein the load voltage has a high value over a high proportion of the period, while a small pulse duty factor results in waveforms having a small proportion. Accordingly, a large power level is produced or output at a large pulse duty factor and a relatively small power level is produced or output at a small pulse duty factor. The pulse duty factor can be adjusted by various control methods. One such method is the so-called phase shift method (rotary control method).
The drive signals of the switching elements of the first half bridge are phase-shifted with respect to the drive signals of the switching elements of the second half bridge during operation of the full bridge in a phase shift method. The phase shift can be half of a period of the alternating output signal. By changing the phase shift, the output signal can be changed. In particular, the power level thereof can be changed.
In principle, it is advantageous for the switching elements to be activated with a voltage which is as small as possible. If the voltage during activation is equal to zero volts, this is called zero voltage switching (ZVS). That is advantageous particularly when the switching element itself has a capacitance at its output. Such capacitance would have to be discharged if the voltage during activation is not equal to zero, which would lead to losses and heating of the switching elements.
In principle, it is also advantageous for the current through the switching element to be as small as possible during deactivation. If the current during deactivation is equal to zero, this is called zero current switching (ZCS). Zero current switching is recommended for switching topologies with leakage inductances in the half-bridge of the switching element and in switching elements which cannot be deactivated quickly, that is to say, in which a relatively large residual current flows, caused, for example, by charge carrier decomposition.
In principle, the objective is to achieve both ZVS and ZCS. However, that is often impossible because the switching times are selected in order to adjust a power level which is to be output and it is often not possible to consider the ZVS, ZCS conditions. For that reason, attempts are made to use the method which is adapted to the switching elements used. This is zero voltage switching (ZVS) in the case of MOSFETs because of the relatively high parallel body capacitance and zero current switching (ZCS) in the case of insulated-gate bipolar transistors (IGBTs) because of the relatively high residual current.
In principle, MOSFETs are preferably used with increasing frequency.
In conventional full bridges/full-bridge circuits, zero current switching is carried out for small pulse duty factors for the switching elements of the first half bridge and zero voltage switching is carried out for the switching elements of the second half bridge.